Suppose the entire population of the world gathers in one spot and, at the sounding of a prearranged signal, everyone jumps up. While all the people are in the air, does Earth gain momentum in the opposite direction? a) No; the inertial mass of Earth is so large that the planet’s change in motion is imperceptible.
b) Yes; because of its much larger inertial mass, however, the change in momentum of Earth is much less than that of all the jumping people.
c) Yes; Earth recoils, like a rifle firing a bullet, with a change in momentum equal to and opposite that of the people.
d) It depends.